Chip Scale Packaging (CSP) is used to form packages that have substantially the same sizes as semiconductor dies. One of the applications of the CSP is the packaging of complementary metal-oxide-semiconductor (CMOS) image sensor dies. The image sensors, which may be formed of CMOS devices, are formed at a front surface of a silicon substrate, which is a portion of an image sensor die. A backside grinding is performed to the silicon substrate until the thickness of the silicon substrate is thin enough to allow light to penetrate the silicon substrate from the backside to reach the image sensors.
In the packaging of the image sensor dies, a glass is bonded to the backside of the image sensor die in order to protect the image sensor die, while at the same time allowing the light to reach the image sensors. The electrical connections to the image sensors are made through T-contacts, wherein aluminum pads over the silicon substrate and electrically coupled to the images sensors are connected to metal lines, which are formed on the sidewalls of the respective package structure.
The packaging of the image sensor dies is performed before the wafers comprising the image sensor dies are sawed. After the ball-grid array (BGA) balls are formed to connect to the T-contacts, the resulting package structure is sawed to separate the image sensor dies. However, since the packaging requires the sawing of aluminum pads, the edges of the aluminum pads are exposed to open air, and are subject to the attack of moisture. This may cause the peeling of the respective aluminum pads. In addition, since the silicon substrate in the image sensor die may be ground to a very small thickness, for example, about 2 μm, the expansion of the aluminum pads caused by the aluminum corrosion will also cause the crack of the silicon substrates, and hence the failure of the resulting package structure.